1. Field of the Invention
The present invention relates to a high frequency transmission cable and, in particular, to a high frequency transmission cable with conductors in the propagation channels thereof being kept juxtaposed and equal in lengths.
2. Description of Related Art
Due to functions of the central processing unit of a computer having been upgraded, the net bandwidth increasing and storage medium data rising tremendously, both the input and output interfaces of the periphery are getting necessary to provide larger bandwidth so that the standard for wide band transmission cables are regulated largely like bamboo shoots after spring. However, problems, such as transmission delay, impedance matching, cross talk, ground noise control and electromagnetic wave radiation interference, resulting from high frequency signals are getting serious in the transmission cables along with the increased bandwidths of the transmission cables.
As for the transmission interfaces in a computer, Serial ATA, the serial transmission equipment, is a transmission cable providing with the highest bandwidth at the present time. Because the Serial ATA has two conductors, i.e., a transmission pair, being used as a transmission channel set for transmitting differential NRZ signal, the conductors in the same transmission channel being very much different from each other in their lengths causes signals transmitted at the same time from the input end being not possible to reach the output end simultaneously in the process of signal transmission. That is, the signals emitted from the input end are possible to be received synchronously.
The difference value of relative propagation delay between two conductors in the field of testing differential signal transmission cables, i.e., the so-called intra pair skew, as shown in FIG. 5, is defined as that a positive signal and a negative signal are transmitted from the input end to the output end via two conductors respectively and transmission time durations t1, t2 for the two signals are measured with the test standard for the transmission based on the signal pulse reaching a middle voltage value and the difference value of the two transmission time durations |t1xe2x88x92t2| is the intra pair skew.
For a conventional cable used for transmitting low frequency signal, the transmission quality affected by inconsistent lengths of the conductors therein is inconspicuous. However, while the frequency of the signal loaded in the transmission cable increases, especially, to more than 1 GHz, the inversed propagation delay between output ends is close to the frequency of the signal and the intra pair skew can affect the quality of the output signal directly. Hence, the influence brought with the intra pair skew is significantly and it becomes a key factor of bandwidth limitation to the conductors.
Referring to FIG. 1, the currently used high frequency cable mainly has two transmission channel sets 10a, 10axe2x80x2 and each of the transmission channel sets 10a, 10axe2x80x2 includes two independent conductors 11a, 11axe2x80x2, a drain wire 30a disposed at two opposite lateral sides of the transmission channel set 10a, 10axe2x80x2, an inner covering 20a providing an effect of shield and wrapping both outer sides of the conductors 11a, 11axe2x80x2 and the drain wire 30a, and a jacket 40a covering the inner covering 20a to constitute a high frequency cable. The inner covering 20a includes a conductive layer 21a and an insulation layer 22a from an inner side to an outer side thereof.
But, the preceding transmission cable has the following problem in practice. For being stored up or delivered handily after having been fabricated, the transmission cable is coiled up. But, the two conductors in the transmission channel easily displace because of being bent during the process of coiling such that it is not easy to control the conductors in their lengths in case of being cut and as soon as the transmission cable is connected to connectors and the transmission channel therein is loaded with high frequency signals, it results in an excessive large intra pair skew due to the two conductors being unequal in their lengths such that it is unable for the output end to receive the signals synchronously and the bandwidth of the transmitting frequency is limited.
Accordingly, an object of the present invention is to provide a high frequency transmission cable to secure the conductors in every propagation channel being kept juxtaposed and equal lengths even if the transmission cable is bent instead of the two conductors being misplaced and unequal as the prior art does so that the propagation delay at the output end can be reduced to enhance the bandwidth of the signal in the transmission cable.
In order to reach the preceding object, the high frequency transmission cable has two conductors in each propagation channel set being wrapped tightly as a single piece to perform communication of electrical appliances in a way of the conductor pair really being juxtaposed and flush with each other for avoiding signals transmitted in the conductors becoming not synchronous due to the conductors being in inconsistent lengths resulting from the cable being bent so as to enhance the frequency width of the transmission cable.